having trouble writing script for bash

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I have to write a program in bash and I understand logically how to do it I just cant get in into code. The problem is this "Write a program called "multiply" to get two numbers from the user and calculate the first number times the second number and show the result.
You are only allowed to use loop and addition operator(+). You are not allowed to use (*)."
I get you have to add one number to the other number nth amount of times to get the "product" but I have no clue how.
Any help?
(HowTos)